Fungicide resistance management strategy for downy mildew (Peronospora destructor) in bulb onion crops in New Zealand

M. Christie 2022; updated 2023, R. Beresford

Disclaimer:

The fungicide and bactericide resistance information in the mode of action group table, the mode of action-based strategies and the crop-based strategies is intended to help manage the effects of pesticide resistance on plant disease control by agrochemicals in New Zealand. The New Zealand Committee on Pesticide Resistance (NZCPR) is a subcommittee of the New Zealand Plant Protection Society (NZPPS). The following parties: NZCPR, NZPPS, any people named in the strategies and any organisation the people may be affiliated with, do not guarantee that the resistance information is complete, consistent, accurate or up-to-date, or that resistance will not develop, or that the degree of resistance will decrease, as a result of the resistance information being used, and the parties accept no liability for loss or damage arising from the use of the resistance information contained in these strategies.

Introduction

Onion downy mildew (ODM) caused by the oomycete pathogen Peronospora destructor lives up to its name being one of the most destructive diseases of onion and onion seed production world-wide. Symptoms often appear first on the older leaves, and if weather conditions are moist and temperatures low, masses of grey to violet fungal spores envelop leaves, which become girdled, collapse, and die. Onion, garlic, and other plants of the allium family all host this disease and spring onions are particularly susceptible. Conditions favouring disease development are prolonged periods of leaf wetness from heavy overnight dew, more than from rainfall.

Fungicide groups at risk from resistance in P. destructor

Fungicide Resistance Action Committee (FRAC) mode of action (MOA) groups that are at risk from resistance and require resistance management for use in ODM control are as follows:

Group 4 Phenyl amide (PA) High risk
Group 7 Succinate dehydrogenase inhibitor (SDHI) Medium to high risk
Group 9 Anilinopyrimidine (AP) Medium risk
Group 11 Quinone outside inhibitor (QoI) High risk
Group 27 Cyanoacetamide-oxime (CAO) Low to medium risk
Group 40 CAA Fungicides Low to medium risk
Group 49 OSBPI Fungicides Medium to high risk

The Group 29 active ingredient, fluazinam, which is co-formulated with the Group 9 fungicide, pyrimethanil, in Apex®, is considered low risk for resistance development. Also classified as low risk are Groups M1, M3, M5, (multi-site inhibitors), Group P07 (plant defence inducers) and Group BM02 (biological control agents). These are all important for use in mixing or alternating with medium to high-risk single-site inhibitor fungicides to delay resistance development. Table 1 lists the fungicides registered for controlling ODM, grouped by FRAC mode of action group code.

**Table 1:** Onion downy mildew fungicides used in New Zealand.
FRAC mode of action group		Active ingredient	Product trade names
4	Phenylamide (PA -fungicides)	metalaxyl	Ridomil® Gold MZ, Speartek® Ventura®
7	SDHI	Boscalid	As part mixture of Pristine®
9	AP fungicides (Anilinopyrimidines)	pyrimethanil	Apex® (combination with fluazinam Group 29)
11	QoI fungicides Quinone outside inhibitors	azoxystrobin	Amistar®, Atlantis, Avior, Azoxystar, Inspire, Roxy, Tazer
		Fenamidone	Reason®
		Pyraclostrobin	Pristine®
		Mandestrobin	Intuity™
27	Cyanoacetamide-oxime	cymoxanil	Curzate® Concord® Curfew®
40	CAA fungicides (Carboxylic acid amides)	Dimethomorph Dimethomorph + mancozeb Dimethomorph + Chlorothalonil Mandipropamid	Sovrin® Flo, Sphinx Acrobat®MZ690 Cobra® Revus®
49	OSBPI Oxysterol binding protein homologue inhibition	oxathiapiprolin	Zorvec® Enicade®
M1	Inorganic	Coppers - various	Various
M3	Dithio-carbamates	Mancozeb	Various
M5	Chloronitriles	Chlorothalonil	Various
P07	Phosphonates	Phosphorous acid	Various
BM02	Microbial	Bacillus sp.	Serenade® Optimum

General resistance management requirements

Observe maximum number of applications and mixing/alternation recommendations for medium and high risk MOA groups and use those groups only when disease levels are low and risk of disease is high, as indicated by available downy mildew weather risk prediction tools.
Use at-risk products preventatively and not curatively and do not use at reduced application rates.
Mixing partners are important for resistance management to reduce pathogen to exposure the at-risk fungicide groups. They must have adequate efficacy against P. destructor and be applied a rate that would provide satisfactory disease control if used alone.
Mixing partners suitable for onion downy mildew include some multi-site products (Groups M1, M3, M5), some biological control agents (Group BM02) and some plant defence inducers (Group P 07) (Table 1).
Use non-chemical management approaches to minimise disease risk, such as avoiding overhead irrigation late in the day and at night, maintaining good soil drainage, destroying volunteer onion plants and crop debris as soon as the crop is harvested. Use crop rotation of at least 2 years and up to four years to minimise risk of carry-over of fungicide resistant pathogen strains.
Not all product labels have adequate resistance management statements and chemical proprietors need to update labels to ensure adequate and accurate information is provided.

Resistance management recommendations by MOA group

Group 4 PA Fungicides

Apply a maximum of 3 PA-containing applications per season preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance.
Apply a maximum of 2 consecutive applications and application intervals should not exceed 14 days and may be shorter when disease risk is high.

Group 7 SDHI fungicides

Apply a maximum of 2 SDHI-containing applications per season preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance.

Group 9 AP fungicides

Apply a maximum of 2 AP-containing applications per season preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance. Pre-mixed formulations with fluazinam are suitable for this purpose.

Group 11 QoI fungicides

Apply a maximum of 4 QoI-containing applications per season or up to 1/3 of the total number of fungicide applications targeting downy mildew, preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance.

Group 27 Cyanoacetamide-oxime fungicides

Apply a maximum of 5 Cyanoacetamide-oxime-containing applications per season and a maximum of 4 consecutive applications, preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance.

Group 40 CAA fungicides

Apply a maximum of 4 CAA-containing fungicide applications per season preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance.

Group 49 OSBPI fungicides

OSBPI-containing fungicide applications should not exceed 1/3 of the total number of fungicide applications targeting downy mildew per season. preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance.